As a background of an optical information carrier, such as an optical magnetic disk or a write-once optical disk, a technique is known in which guide grooves are provided on the optical information carrier and are utilized to control the tracking of beams for recording and reproduction of information. A conventional system mainly used as a tracking information detecting system is called a push-pull system. In the push-pull system, tracking information is obtained by using one light beam and detecting a difference in intensity between .+-. 1st order diffracted lights which are produced when the light beam extends across the guide groove.
However, since the push-pull system utilizes a change in light amount distribution of a far field pattern of reflected light from the optical information carrier corresponding to the difference in intensity between the .+-. 1st order diffracted lights which is produced in accordance with the degree of extension of the light beam across the guide groove, this system is greatly affected by the inclination of the optical information carrier with respect to the optical axis of beam, the form of the guide groove, the deviation of the optical axis of lenses from the optical axis of beam, the positional deviation of light detectors and so on. Therefore, the requirements for the mechanical characteristic of the optical information carrier are severe. As for an optical information recording/reproducing system, sufficient attention must be paid to the precision of assembling/adjusting an optical head, the positional deviation of parts attendant upon the lapse of time, and so on. In addition, even if sufficient attention is paid, it is difficult to reduce the offset of the light beam with respect to the center axis of the track into zero. Also, in order to minimize a lens shift, a two-stage servo system is employed in which a control is made interlocking a coarse actuator and a fine actuator with each other. There results in an optical information recording/reproducing system which is complicated and high in cost.
As a system for solving the above problems there is known a three-beam system in which three beams are used. In the following, the conventional optical information recording/reproducing system for an optical magnetic disk using the three-beam system will be explained by use of FIG. 4. In the figure there is seen a laser diode, a collimator lens 2, a diffraction grating 3, beam shaping prisms 4 and 5, beam splitters 6 and 7, a riser mirror 8, a focusing lens 9, a 1/2 wavelength plate 10, a polarized beam splitter splitter 11, a mirror 12, a converging lens 13, a cylindrical lens 14, light detectors 15 to 17, and an optical magnetic disk 18.
In FIG. 4, a laser beam emitted from the laser diode 1 in a horizontal direction is collimated by the collimator lens 2 and is then developed by the diffraction grating 3 into a zero order diffracted light and .+-. 1st order diffracted lights. Each of those diffracted lights is shaped by the beam shaping prisms 4 and 5 into a light beam having a circular spot form. The zero order diffracted light will be called a main light beam, and the .+-. 1st order diffracted lights will be called subsidiary light beams. Also, the main and subsidiary light beams will generally be termed a light beam.
The light beam shaped by the beam shaping prisms 4 and 5 into a circular spot form is passed through the beam splitters 5 and 6, risen by the riser mirror 8 in a vertical direction and focussed onto the optical magnetic disk 18 by the focusing lens 9.
The light beam impinging upon the optical magnetic disk 18 is reflected by the optical magnetic disk 18. The light beam with which the optical magnetic disk 18 is irradiated is a P-polarized beam having a plane of polarization parallel to an information track on the optical magnetic disk 18. When an area of the optical magnetic disk 18 having information magnetically recorded thereon is irradiated with such a light beam, the plane of polarization of the light beam is rotated by a Kerr effect in accordance with the magnitude of magnetization of the information area so that a P-polarized component and an S-polarized component having a plane of polarization perpendicular to an optical axis of the light beam are produced. Namely, a reflected light beam includes a P-polarized component and an S-polarized component and the proportion of the P-polarized component and the S-polarized component to each other differs depending upon the magnitude of the Kerr effect.
The reflected light beam from the optical magnetic disk 18 reaches the beam splitter 7 through the focusing lens 9 and the riser mirror 6. The beam splitter 7 reflects the whole of the S-polarized component of the reflected light beam and a part of the S-polarized component thereof. The reflected light beam thus reflected by the beam splitter 7 is rotated in a plane of polarization by 45.degree. by the 1/2 wavelength plate 10 and is then split by the polarized beam splitter 11 into a P-polarized component and an S-polarized component. The amount of light of a P-polarized component of the zero order diffracted light beam is detected by the light detector 16, and the amount of light of an S-polarized component of the zero order diffracted light passed through the polarized beam splitter 11 and reflected by the mirror 12 is detected by the light detector 17. An information signal magnetically recorded on an information track of the optical information carrier 18 is obtained by producing a difference between output signals of the light detectors 16 and 17, and a signal modulated by pre-pits from brightness to darkness produced by pre-pits on the optical information carrier 18 is obtained by producing a sum of the output signals of the light detectors 16 and 17.
A part of the P-polarized component of the reflected light beam transmitted through the beam splitter 7 is reflected by the beam splitter 6 and is then converged by the converging lens 13. The reflected light beam converged by the converging lens 13 is subjected to astigmatism by the cylindrical lens 14 and is received by the light detector 15. The light detector 15 is composed of six segmental photo diodes. Four of the six photo diodes receive the zero order diffracted light in the reflected light beam having been subjected to astigmatism. Focusing information is obtained by operationally processing output signals of four of those photo diodes. One of the two remaining photo diodes of the six photo diodes receives the + 1st order diffracted light in the reflected light beam having been subjected to astigmatism and the other photo diode receives the - 1st order diffracted light. Tracking information is obtained by producing a difference between output signals of two of those photo diodes.
According to the three-beam system explained above, the detection of the tracking information uses only a difference in amount of light between the .+-. 1st order diffracted lights. Therefore, the system is hardly affected by the inclination of the disk with respect to the optical beam axis. Also, it is only required that focus spots of the .+-.1st order diffracted lights reflected fall off the photo diodes which receive the .+-. 1st order diffracted lights. Accordingly, the positional adjustment of those photo diodes is easy.
In the case where an optical information carrier is used for recording of code data, it is general that pre-pits are formed on the optical information carrier. In one example, a header portion indicative of address information is formed as pre-pits for every sector. At a location where such pits exist, variations in amount of reflected light of the .+-. 1st order diffracted lights are produced and have an influence upon the precision of detection of tracking information. However, since the length of the header portion is sufficiently short as compared with that of an area in which data is recorded (or a data area), the influence is not very large.
On the other hand, there may be the case where a data area includes a so-called ROM region in which data is beforehand recorded by pre-pits. When data is reproduced from such a ROM region, the amounts of reflected lights of the .+-.1st order diffracted lights vary owing to the pre-pits over a long time as compared with the case where pre-pits are formed in only the header portions. As a result, it becomes impossible to obtain correct tracking information. The influence of pre-pits upon the detection of tracking information will now be explained by use of FIG. 5.
FIG. 5(a) shows guide grooves on an optical information carrier and individual light beams involved in the three-beam system. Data is recorded on a track between the guide grooves. The irradiation of the optical information carrier with .+-. 1st order diffracted light is made with the .+-. 1st order diffracted lights being shifted by one-fourth of the track pits from a zero order diffracted light on opposite sides of the zero order diffracted light in a direction perpendicular to the track (or an arrow X direction) and being shifted by about 40 .mu.m (when the track pitch is 1.6 .mu.m) from the zero order diffracted light before and behind the zero order diffracted light in a track direction (or an arrow Y direction).
It is assumed that the + 1st order diffracted light is positioned on the right side of the zero order diffracted light and the - 1st order diffracted light is positioned on the left side thereof, when seen in the arrow X direction. The amount of reflected light of each light beam becomes the maximum when the center of that light beam coincides with a center line of the track and becomes the minimum when the center of the light beam coincides with a center line of the guide groove. Therefore, in the case where the optical information carrier is rotated so that the zero order diffracted light and the .+-. 1st order diffracted lights are moved in the arrow X direction, the amounts of reflected lights of the .+-. 1st order diffracted lights as taken at the position of the center of the zero order diffracted light exhibit changes as shown in FIG. 5(b). Namely, though the amount of reflected light of each of the .+-. 1st order diffracted lights changes with a period equal to the track pitch, the change of the amount of reflected light of the .+-. 1st order diffracted light is in advance of that of reflected light of the zero order diffracted light by a time of the one-fourth of that period and the change of the amount of reflected light of the - 1st order diffracted light is delayed from that of reflected light of the zero order diffracted light by a time of the one-fourth of that period.
The change of reflected light of each light beam shown in FIG. 5(b) corresponds to the case where pre-pits are not formed on the track. When the center of the zero order diffracted light coincides with the center line of the track, the amounts of reflected lights of the .+-. 1st order diffracted lights become equal to each other. When the center of the zero order diffracted light is shifted from the center line of the track by the one-fourth of the track pitch, a difference in amount of reflected light between the .+-. 1st order diffracted lights becomes the maximum.
The above concerns the case where the irradiation is made of a portion where pre-pits are not formed on the track. In the case where the irradiation is made of a portion where pre-pits are formed, the intensity of the reflected light of each light beam is modulated by the pre-pits. This situation is shown in FIGS. 5(c) and 5(c') with reference to a time axis t.
FIG. 5(c) shows a change of the amount of reflected light of the + 1st order diffracted light and FIG. 5(c') shows a change of the amount of reflected light of the -1st order diffracted light. In each figure, a broken line represents a change of the amount of reflected light in the case where there is no pre-pit.
Even in the case where pre-pits exist, the amount of reflected light of each of the .+-. 1st order diffracted lighcs is expected to take the maximum value when the center of that diffracted light coincides with the track center line. However, this holds for the case where an interval between pre-pits or the density of pre-pits is fixed. In actual, the density of pre-pits changes depending upon the contents of information. As the density of pre-pits becomes higher, the effect of interference of light becomes larger so that the amount of reflected light is lowered. Therefore, even if the center of each of the .+-. 1st order diffracted lights coincides with the track center line, it does not necessarily follow that the amount of reflected light of that diffracted light becomes the maximum. Also, even if the center of the zero order diffracted light coincides with the track center line, it does not necessarily follow that the amounts of reflected lights of the .+-. 1st order diffracted lights become equal to each other.
In order to eliminate the influence of prepits, there may be considered a method in which the output signal of each of light detectors to receive the reflected lights of the .+-. 1st order diffracted lights is passed through an LPF (low pass filter) to extract an average value (or a DC component) without modulated components caused by the pre-pits and the levels of the extracted DC components for the .+-. 1st order diffracted lights are compared to obtain tracking information.
However, the obtained DC component also changes in accordance with the modulation by the prepits or in accordance with the density of pre-pits, as shown as the average value by a solid line in FIG. 5(c) or 5(c'). Therefore, a tracking information signal obtained by comparing the levels of the DC components results in a solid line shown in FIG. 5(d). Namely, a normal tracking information signal cannot be obtained, as apparent from the comparison of the solid line of FIG. 5(d) with a broken line of FIG. 5(d) which represents a correct tracking information signal obtained from the amounts of reflected lights of the .+-. 1st order diffracted lights shown by the solid lines in FIG. 5(b).